The present invention relates to a motor-driven rotary tool (e.g., an electric screwdriver) driven by an electric motor, and particularly relates to a motor-driven rotary tool equipped with an internal heating temperature detection function that is configured such that, when the electric motor housed within the holding part is used continuously over an prolonged period, to prevent injury (e.g., low-temperature burn) to the operator as the result of the increase in the temperature at the holding part of the motor-driven rotary tool resulting from the heating of the electric motor, a temperature sensor is provided in the casing, or the like of the electric motor to detect an abnormal temperature increase in the casing, or the like as the result of the heating of the electric motor and to then shut down the power-supply circuit of the electric motor, thereby stopping the driving of the motor-driven rotary tool.
Conventionally, as a motor-driven rotary tool (e.g., an electric screwdriver) that is driven by means of an electric motor, a motor-driven rotary tool equipped with an automatic stopping means configured as follows is known: The driving of the electric motor performs such work as screwing by driving a rotary tool such as a screwdriver bit; the rotary tool is equipped with a torque detection means that detects the load torque that occurs in the rotary tool during the work specified previously; and after the load torque reaches the preset torque, the aforementioned torque detection means detects this state and stops the driving of the rotary tool.
For example, an electric screwdriver or the like, configured as follows has been proposed and implemented: When a screw or the like is driven and a strong opposing load is exerted on the driver bit, a clutch mechanism that operates at a preset tightening torque detects that a specified torque has been reached, after which the aforementioned clutch mechanism is activated to temporarily break the connection between the electric motor's output shaft and the driver bit. Furthermore, an electric screwdriver (Japan Patent Application Publication No. 60-13798) or the like that also has been developed and put to practical use is configured such that, when the aforementioned clutch mechanism is activated, a limit switch or the like detects this state and stops the driving of the electric motor.
When a switching circuit is provided in the power-supply circuit of an electric screwdriver, or the like, having a driver bit that is rotary-driven by an electric motor, and the load current that flows in the power-supply circuit as the aforementioned electric motor is driven reaches the specified tightening torque near the end of the tightening, the load current increases to an overload current above a specified value. Therefore, an electric screwdriver (Japan Patent Application Publication No. 57-43389) or the like equipped with an automatic power shutdown device and configured as follows also was provided: The overload current state is detected and the supply of driving current to the electric motor is cut off, and after a given amount of time elapses, the supply of driving current to the electric motor is restored. Moreover, a switch mechanism is provided in the armature circuit of the electric motor, and the aforementioned electric motor armature circuit is short-circuited when the aforementioned switching circuit cuts off the supply of driving current to the electric motor, thereby instantaneously stopping the electric motor by means of regenerative braking. Furthermore, the aforementioned switching circuit transitions to the OFF state only for a given amount of time, so the following functionality is provided: Regarding the timing for performing the next screw-tightening operation, the switching circuit is restored automatically and transitions to the ON state, so it is possible to drive the electric motor and immediately begin the next screw-tightening operation.
However, in this type of motor-driven rotary tool, when the operator continuously uses the electric motor for a long time during manual labor, the electric motor heats and the temperature increase of the casing or the like is transmitted to the holding part of the motor-driven rotary tool, thereby risking a so-called low-temperature burn caused by the inflammation that reaches the surface and interior of the skin of the operator's hand. That is, it was ascertained that, even when the temperature of the holding part directly contacted by the operator's hand is within the range 45° C. to 65° C., continuous, prolonged contact (e.g., at least 2 hours) results in a low-temperature burn such that the degree of inflammation of the surface and interior of the contacting skin increases with the contact time, and considerable effort and time are required for therapy and recovery.
Therefore, as a result of assiduous study and a series of prototypes, the inventors confirmed that, when the temperature of the holding part of a motor-driven rotary tool directly contacted by the operator's hand reaches a temperature at which a low-temperature burn occurs readily (i.e., 45° C. to 65° C.), the temperature of the casing or the like resulting from the heating of the internal electric motor usually has risen to 55° C. to 75° C. Based on this perspective, the inventors were able to determine that, when a temperature sensor is installed in the casing or the like of an electric motor housed within the holding part of the aforementioned motor-driven rotary tool and the temperature of the casing or the like of the aforementioned electric motor reaches 55° C. to 75° C. or higher, if the rotary tool is provided with means such that this state is detected by the aforementioned temperature sensor and the power-supply circuit of the electric motor is shut down, thereby stopping the driving of the electric motor, it is possible to interrupt the operator's work by stopping the operation as a motor-driven rotary tool, thereby conveniently and easily preventing the occurrence of injury caused by low-temperature burn.
Consequently, the purpose of the present invention is to provide a motor-driven rotary tool equipped with an internal heating temperature detection function that enables the convenient and easy prevention of injury (e.g., low-temperature burn) to the operator caused by the heat of the electric motor housed within the holding part of the motor-driven rotary tool.